The present invention relates to a scanning electron microscope to observe a fine pattern to measure its dimensions and more particularly a scanning electron microscope designed to take measurements of samples whose shape changes as they are applied with an electron beam.
In a manufacturing and inspection process of functional element products, such as semiconductor devices and thin-film magnetic heads, which are fabricated by a surface microfabrication technique, a scanning electron microscope is widely used for measurement of widths of patterns fabricated (referred to as “critical dimension measurements”) and for external inspection. The scanning electron microscope forms an image of a sample in a sequence of steps that involves: linearly or two-dimensionally scanning an electron beam, which is emitted from an electron source and finely focused by a converging lens or objective lens using interactions between a magnetic field or electric field and the electron beam, over the sample using a beam deflector; detecting a secondary signal (secondary electrons and back-scattered electrons) produced by the electron beam by using a photoelectric effect-based detector; and transforming the detected signal into a visible signal, such as a luminance signal synchronous with the scanning of the electron beam.
In scanning electron microscopes, provisions are made to ensure that an image of a specimen obtained depicts, with high precision, features of a specimen surface being observed and measured. That is, when the specimen surface is examined, visualization points of the image signal are arranged at positions precisely similar to those positions of the corresponding points in an area being scanned. To realize this position matching, the scan area and the image area are both set rectangular in shape and made up of the same number of scan lines whose length is equal to one side of the rectangle. Generally, the scan area and the image area are set to have the same ratio of a scan line length to a distance between the adjoining scan lines. With this arrangement, a distance between any two points on the specimen surface has a constant ratio to a distance between the corresponding two points in the image of the specimen at all times. This ratio signifies a magnification of the scanning electron microscope. Such a technology has been widely implemented as a basic technology in the scanning electron microscopes, as described in L. Reimer, “Scanning Electron Microscopy”, Springer-Verlag Berlin and Heidelberg GmbH & Co. KG, 1985, page 2, for example. From the specimen image thus obtained, the distance between any two points on the specimen surface can be calculated easily. This calculation is generally called “critical dimension measurement” and a scanning electron microscope with such a calculation function is called a “critical dimension scanning electron microscope.”
An example case where the scan area on the surface of a specimen and the corresponding specimen image are not similar is described in JP-A-2001-147112. In this example, for a specimen with patterns so spaced apart as to make it necessary to measure dimensions with a reduced magnification although the patterns are very fine, a secondary electron image is formed by extending the specimen image in a direction perpendicular to a line connecting two points on the specimen, thereby improving the measurement accuracy.
Such a scanning electron microscope therefore needs to radiate against the surface of the specimen being probed an electron beam with a surface arrival energy of several hundred electron bolts.
As features of microfabricated semiconductor surfaces have become more and more miniaturized in recent years, a photoresist that reacts to an argon fluoride (ArF) excimer laser beam (referred to as an “ArF resist”) has come to be used as a photosensitive material for photolithography. The ArF laser beam has a short wavelength of 193 nm and thus the ArF resist is considered suited for exposing finer circuit patterns. Our recent study, however, has found that the ArF resist is very fragile when subjected to electron beams and that, when a fabricated pattern is probed or its critical dimensions are measured by a scanning electron microscope, a base material such as acrylic resin is condensed by the focused electron beam, reducing its volume (referred to as a “shrink”) and deforming the circuit pattern.
To realize a design performance of semiconductor devices requires a stringent control of shapes and dimensions of circuit patterns and, for this reason, a critical dimension scanning electron microscope capable of measuring very fine dimensions is used in an inspection process. However, if the shape of a pattern changes upon application of a critical dimension measuring electron beam during the observation and measurement processes, an expected design value of the circuit pattern dimension cannot be realized, leading to a problem of degradation of device characteristics and failures. Further, since line widths change when subjected to an electron beam, a measured value of the same dimension may vary each time a measurement is made, making it impossible to improve the measurement accuracy. At present, no other equipment than the critical dimension scanning electron microscope is available that can measure fine dimensions at a satisfactory precision. So, the pattern shrink poses a serious problem to the semiconductor device manufacture using an ArF resist. Conventional scanning electron microscopes, as described above, do not provide a means to deal with the shrink of a specimen caused by the electron beam and there is a problem with measured values of pattern dimensions. Further, in the case of the JP-A-2001-147112 described above, although the measurement accuracy of a dimension between two separate points on a specimen is taken into consideration, no provisions are made to deal with the shrink of specimen caused by the application of an electron beam.
It is an object of this invention to provide a scanning electron microscope capable of measuring a pattern formed on a specimen that shrinks upon being radiated with an electron beam, such as an ArF resist.
The shrink of an ArF resist pattern is considered to be a chemical reaction caused by the energy of a focused electron beam incident on the resist. It is thus considered that the volume of the ArF resist will change after the irradiation of electron beam according to an equation (1) below.Vs=V exp(−t/τ)   (1)where Vs is a volume of the ArF resist after being irradiated with an electron beam, V is a volume of the ArF resist before the application of the electron beam, τ is a time constant of the chemical reaction in the ArF resist, and t is an elapsed time.
As can be seen from equation (1), there is a time lag from when an electron beam is radiated against the ArF resist until the resist shrinks by a reaction. When a scanning electron microscope is used to make measurements, since an image formed in only one scan (1 frame) has a poor S/N ratio, it is common practice to overlap a plurality of frames and produce an averaged image to improve the S/N ratio and thereby enhance the measurement accuracy. Based on the fact that (1) there is a time lag from an instant the ArF resist is irradiated with an electron beam until a reaction takes place and that (2) normally a plurality of frames are overlapped to form a target image for dimension measurement, we have studied measures to deal with the problems described above.
Suppose a time which elapses from when an electron beam scan is performed once over a specimen until the next scanning electron beam reaches the same position on the specimen is Ti and that a time constant of the chemical reaction of the ArF resist is τaccording to equation (1). In an interlaced scan at 60 Hz, Ti was 33 ms and, in an experiment performed by the inventor, τ was 5 ms. At this time, Ti>τ and the second electron beam scan is executed after the shrink of the ArF resist has progressed significantly (see FIG. 1A). On the contrary, this invention has Ti<<τ (see FIG. 1B). By shortening the scan interval, it is possible to make precise measurements with a reduced amount of shrink.
That is, a highly accurate measurement is made of a resist such as an ArF resist that shrinks when subjected to an electron beam, by changing a scanning order of scan lines to shorten the time between the first and the second scan over the same location on the specimen to allow the scans to be performed successively while the amount of shrink is small. The time between the scans can also be shortened by reducing the number of scan lines or shortening the scan width. Further, a user is provided with an environment that makes for an easier use of the microscope by registering parameters, such as scanning order, the number of scan lines, scan width and the total number of frames, as fixed values in advance so that the user can choose a desired combination of these at time of taking measurements.